The already shaky U.S.-Pakistan relationship is set to be rocked by revelations that Pakistan gave China access to the previously unknown U.S. "stealth" helicopter that crashed during the commando raid that killed Osama bin Laden in May.

The move by the Pakistanis came despite requests from the CIA not to let China see them.

The disclosure by the Financial Times on Sunday, if confirmed, will put a dent in the slowly improving relationship after hitting its lowest point in decades following the killing of bin Laden.

Secret weapon: The modified Black Hawk that crashed inside bin Laden's compound and that was reportedly shown to Chinese officials

During the raid, one of two modified Blackhawk helicopters, believed to employ unknown stealth capability, malfunctioned and crashed, forcing the commandos to abandon it.

'The U.S. now has information that Pakistan, particularly the Inter-Services Intelligence Directorate (ISI), gave access to the Chinese military to the downed helicopter in Abbottabad,' the paper quoted a person 'in intelligence circles' as saying on its website.

It said Pakistan, which enjoys a close relationship with China, allowed Chinese intelligence officials to take pictures of the crashed aircraft as well as take samples of its special 'skin' that allowed the American raid to evade Pakistani radar.

Checking it out: A Pakistani Army soldier inspects part of the wreckage

One U.S. official, speaking on condition of anonymity, told Reuters there was reason to believe Pakistan had allowed the Chinese to inspect the aircraft.

But the official could not confirm it happened with certainty.

No one from the Pakistani army was available for comment, but the ISI, Pakistan's top spy agency, denied the report.

The paper said Pakistan's top general, chief of army staff Ashfaq Kayani, denied that China had been given access.

The surviving tail section, photos of which were widely distributed on the internet, was returned to the U.S. following a trip by U.S. Senator John Kerry in May, a spokesman for the U.S. embassy told Reuters.

Shortly after the raid, Pakistan hinted that it might give China access to the helicopter, given its fury over the raid, which it considers a grievous violation of its sovereignty.

Abandoned: The commandos were forced to leave the chopper untouched in the fast and furious raid

A total mess: Part of the wreckage of the Blackhawk. The tail section was eventually returned to the U.S.

'We had explicitly asked the Pakistanis in the immediate aftermath of the raid not to let anyone have access to the damaged remains of the helicopter,' the Financial Times quoted the source as saying.

In an incident such as the helicopter crash, it is standard American procedure to destroy sophisticated technology such as encrypted communications and navigation computers.

Pakistan is a strategic ally to the U.S. but the relationship has been on a downward spiral since the killing of the al Qaeda leader in the raid by U.S. forces.

Curiosity: Local media and residents gather around the bin Laden compound in Abbottabad in the days following May's raid

Islamabad was not informed in advance and responded by cutting back on U.S. trainers in the country and placing limits on CIA activities there.

The fact that the al Qaeda chief lived for years near the Pakistani army's main academy in the north western garrison town of Abbottabad reinforced suspicions in Washington about Islamabad's reliability in the war against militant Islamists.

There are also growing frustrations with Pakistan over its reluctance to mount offensives against militant factions in the north west who are fighting U.S.-led foreign forces across the border in Afghanistan.

Despite the billions in aid, Pakistan still considers China a more reliable ally than the United States.

China is a major investor in predominantly Muslim Pakistan in areas such as telecommunications, ports and infrastructure.

The countries are linked by a Chinese-built road pushed through Pakistan's northern mountains.